Add the source content for the out-of-tree build test
under `apps/testing/cxx-oot-build`. This supports the
new CI check in NuttX to prevent regressions in the
`make export` workflow.
The test project provides:
* Sample OOT build structure.
* Integration with exported `nuttx-export`.
* Verification of successful build and link.
Signed-off-by: trns1997 <trns1997@gmail.com>
